Short answers, pulled from the story.
More than a million insect species have been described and named, and estimates suggest perhaps 5.5 million exist in total. Insects represent more than half of all animal species and around half of all eukaryote species.
Insects are hexapod invertebrates with a chitinous exoskeleton, a three-part body of head, thorax and abdomen, three pairs of jointed legs, compound eyes, and a single pair of antennae. Adult insects are also the only arthropods that ever have wings, with up to two pairs.
The word insect comes from the Latin insectum, from in and seco meaning cut up, because insects appear cut into three parts. Pliny the Elder introduced it, calquing the Greek entomon, and the English word first appears in 1601 in Philemon Holland's translation of Pliny.
Insects breathe without lungs, taking air in through paired spiracles along the sides of the abdomen and thorax. Tracheae and tracheoles deliver oxygen directly to the tissues, so the blood does not carry oxygen and the circulatory system is greatly reduced.
The respiratory system limits insect size, because gas exchange through spiracles becomes less efficient as insects get larger. The heaviest insect today weighs less than 100 grams, though higher atmospheric oxygen in the late Paleozoic allowed dragonflies with wingspans of more than 2 feet.
Humans rely on insects for pollination, valued at about $34 billion in the United States in 2021, and for products including honey, wax, silk and carmine. Insects are eaten as food in 80% of the world's nations, and the fruit fly Drosophila melanogaster is a key laboratory model organism.